1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and particularly to a process of planarizing a wafer.
2. Description of the Prior Art
There are several different types of semiconductor devices, for example memory devices, electronic signal processors, devices for capturing or acquiring images, etc. In a semiconductor process, it is common to carry out a planarization on a rough semiconductor surface to enhance precision of pattern transfer. The planarization is usually accomplished by a chemical mechanical polishing (CMP) method. The theory of CMP is to place a semiconductor wafer on a polishing station, then add a slurry to use chemical reaction and mechanical polishing to planarize a rough semiconductor surface. Dry etch is also a common process to chemically remove layers from the surface of a wafer during manufacturing. During etching, part of the wafer is protected from the etchant by a mask which resists etching. The mask may be a patterned photoresist or a durable mask, such as silicon nitride.
For more specific examples, such as 45 nm DRAM and smaller, the fabrication of container capacitor module starts with a standard oxide layer, the oxide is subsequently removed from the main array and any other areas where capacitors will be formed, and then those areas are filled with polysilicon. Deep contacts are formed in such polysilicon. The conductive stack must be later removed (or not placed) in the DRAM periphery logic areas. This process flow requires a challenging CMP process for large step (1.5-2.0 μm) replanarization of the polysilicon layer. In such CMP process, dishing phenomenon tends to occur in the array region, which may be illustrated by the memory cell array region 203 including polysilicon surrounded by the peripheral logic region 204 including oxide in FIG. 1.
Therefore, there is a need for a novel process of planarizing a wafer, during which there is a large step height and/or surface area between a memory cell array region and a peripheral region, without occurrence of dishing phenomenon.